1. Field of the Invention
The present invention relates to a membrane electrode assembly (also referred to as an MEA), a manufacturing method thereof and a fuel cell. In particular, the present invention relates to an MEA which has a high level of power generation performance, and a manufacturing method thereof and a polymer electrolyte fuel cell (PEFC) or proton exchange membrane fuel cell (PEMFC) using the MEA.
2. Description of the Related Art
Fuel cells are power generation systems which produce electric power along with heat. A fuel gas including hydrogen and an oxidant gas including oxygen reacts together at electrodes containing catalyst so that the reverse reaction of water electrolysis takes place in a fuel cell. Fuel cells are attracting attention as a clean energy source of the future since they have advantages such as a small impact on the environment and a low level of noise production relative to conventional power generation systems. Fuel cells are divided into several types according to the employed ion conductor. A fuel cell which uses an ion-conductive polymer membrane is called a polymer electrolyte fuel cell (PEFC) or proton exchange membrane fuel cell (PEMFC).
Among various fuel cells, a PEFC (or PEMFC), which can be used at around room temperature, is considered as a promising fuel cell for use in a vehicle and a household stationary power supply etc. and is being developed widely in recent years. A joint unit which has a pair of electrode catalyst layers on both sides of a polymer electrolyte membrane and which is called a membrane electrode assembly (MEA) is arranged between a pair of separators, on which either a gas flow path for supplying a fuel gas including hydrogen to one of the electrodes or a gas flow path for supplying an oxidant gas including oxygen to the other electrode is formed, in the PEFC (or PEMFC). The electrode for supplying a fuel gas is called a fuel electrode or cathode (electrode), whereas the electrode for supplying an oxidant gas is called an air electrode or anode (electrode). Each of the electrodes includes an electrode catalyst layer, which has stacked polymer electrolytes with carbon particles on which a catalyst such as a noble metal of platinum group is loaded, and a gas diffusion layer which has gas permeability and electron conductivity.
Conventionally, various manufacturing methods of membrane electrode assembly have been studied to improve the fuel cell performance. Examples of the manufacturing method of membrane electrode assembly include a method in which a catalyst layer is formed as an electrode by coating a coating liquid containing a catalyst onto the ion-exchange membrane and the electrode and the ion-exchange membrane are joined by a heat treatment such as hot press to make the membrane electrode assembly, a method in which a catalyst layer is formed on a substrate film that is prepared independently of an ion-exchange membrane and the ion-exchange membrane is stacked on the catalyst layer to transfer the catalyst layer onto the ion-exchange membrane by the hot press, a method in which an electrode sheet in which the catalyst layer is formed is prepared on a gas diffusion layer to join the electrode sheet to the ion-exchange membrane, and a method in which two sets of half cells in which the catalyst layer is formed on the ion-exchange membrane are prepared, surfaces of the ion-exchange membrane sides are pressure-bonded while faced to each other, thereby manufacturing the membrane electrode assembly.
<Patent document 1> JP-A-2003-197218
<Patent document 2> JP-A-2005-294123
<Patent document 3> JP-A-2005-108770
The membrane electrode assemblies manufactured by these methods, however, are made using a heat press such as hot press etc. for combining the electrode catalyst layers with the ion-exchange membrane. Since such a heat press becomes a critical (bottle-neck) process and causes an increase in the tact time, there is a problem of fall of production efficiency.
In addition, membrane electrode assemblies manufactured by the heat press such as hot press etc. is liable to be dried up because the moisture is easily lost from the cross sectional surfaces of the catalyst layer, which is left exposed and uncovered with the catalyst layers unlike the facing surface, which is covered with the membrane. As a result, there is also a problem of decrease in power generation performance under a low humidified condition in which only moisture produced by the fuel cell reaction is supplied. Membrane electrode assemblies which operate on such a low humidified condition, however, will rather be required in the future.
<Patent document 1> to <Patent document 3> are examples of reporting sequentially stacked MEAs in which a first electrode catalyst layer is formed on a substrate followed by forming a polymer electrolyte layer and a second electrode catalyst layer. Although descriptions relating to improving production efficiency are written in these examples, improvement of moisture retention capability and power generation performance under a low humidified condition have not been achieved in these examples.